Similar to Kamen Rider Decade and his Kamen Ride forms, Build transforms into Ex-Aid's primary form, the Kamen Rider Ex-Aid Action Gamer Level 2 and is equipped with the Gashacon Breaker, which he can dual weild with his Drill Crusher. It is still unknown if this is the only Legend Rider form Build will transform in said film.
Kamen Rider Heisei Generations FINAL is set to premiere on December 9th.
[Source: HeroShock, Kamen Rider Wiki]